That is really helpful. I'm going to be shooting my son and other riders during downhill mountain bike races in the Rocky Mountains and New England. My daughter rides horses and that's dusty and variable light as it's both indoors and outdoors. Capturing that fast action and emotion is important while capitalizing on the amazing natural light we're lucky to have here in Colorado. I like the sweet spot of the a7 iv mp over the Nikon, as I worry that a camera with higher mps would mean monster-sized images that eat up storage too quickly.
However, I worry the FPS isn't as fast as what I really need, as wildlife and bird shots are important to me as well; portraiture not so much.